Mechanisms of reproductive aging in the male are studied using Sprague Dawley rats as animal models. Experiments were conducted to examine testicular production of testosterone and estrogen under basal conditions and gonadotropin (hCG) stimulation. Results indicated that aged rats showed a 400 percent decrease in testosterone secretion at basal conditions, and under saturating doses of hCG stimulation, the aged rat responded positively so that the differences amount only to a 70 percent decrease when compared to the young. Both serum level of LH and the number of hCG receptors in the testes do not show any age differences when young and old rats are compared. Testicular production of estrogens appears higher in the 4m rats than 18-month old rats and this is reflected both by the secretion into testicular venous blood and in vitro conversion from exogenous androgens. Peripheral blood levels of estradiol, however, remain at similar level regardless of age, suggesting increased peripheral aromatization in the aged rats.